Electro-optical lenses that utilize birefringent liquid crystal to alter their optical power are well known. They have the inherent advantage over conventional glass or plastic optical lenses of being able to switch their optical power by the application of an electric field. One drawback of many liquid crystal electro-optic lens designs is that a large number of lithographically etched transparent electrodes are often required to create the desired phase profile.
One basic structure of electro-optic liquid crystal lenses is that of a thin layer of liquid crystal sandwiched between two transparent substrates. Onto the inner surfaces of each substrate, a transparent metallic electrode structure is formed. An alignment layer is formed on top of the electrode layers to establish a specific orientation, i.e., pre-tilt, of the liquid crystal molecules when there is no electric field present. An electric field is established across the liquid crystal layer when voltage is applied to one electrode layer and an electric potential is created between the electrodes. If the electrode structure is patterned, a spatially varying voltage can be applied to the electrodes, and a gradient in the field is created that gives rise to a gradient in the index of refraction of the liquid crystal layer. With proper design of the electrode structure and the applied voltages, an electro-optic lens can be fabricated. Electro-optic converging lenses can be made with large retardance in the lens center, and falling off radially. Conversely, a diverging electro-optic lens has maximum retardance at the perimeter and minimum at the center.
Several references disclose methods of pre-tilt and head-on birefringence control and various optical devices that utilizes the methods, see for example, R. A. Kikmet and C. de Witz, “Gel Layers For Inducing Adjustable Pre-tilt Angles In Liquid Crystal Systems,” J. Appl. Phys., Vol. 20, No. 3, 1991, pp. 1265-1269; S. Masuda, T. Nose and S. Sato, “Optical Properties of a UV-Cured Liquid Crystal Microlens Array,” Applied Optics, Vol. 37, No. 11, 1998, pp. 2067-2073; H. Ren and S.-T. Wu, “Tunable Electronic Lens Using a Gradient Polymer Network Liquid Crystal,” Appl. Phys. Lett., Vol. 82, No. 1, 2003, pp. 22-24; and H. Ren, Y.-H. Fan, S. Gauza and S.-T. Wu, “Turnable Microlens Array Using Polymer Network Liquid Crystal,” Opt. Commun., Vol. 230, 2004, pp. 267-271; and R. A. Kikmet and C. de Witz, “Gel Layers For Inducing Adjustable Pre-tilt Angles In Liquid Crystal Systems,” J. Appl. Phys., Vol. 20, No. 3, 1991, pp. 1265-1269.